Many fear the virus, known as Bagle, could be a prelude to a broader series of attacks or a fresh spam effort.

Although the virus – known as a worm because it replicates itself and sends out more copies via e-mail - does not appear to damage computers, experts are concerned that it installs a programme on the computers of users who open the attachment.

This could enable the virus authors to send out barrages of e-mail advertising that could generate money.

“It seems perfectly possible that Bagle is yet another worm written by spammers,” Mikko Hypponen of the Finnish-based security firm F-Secure said.

“This way, they could first infect a large amount of computers. When they have enough, they could automatically install invisible e-mail proxy servers on each machine and start spamming through them.”

Medium risk

Network Associates, another security firm, called the virus a “medium risk”.

“The Bagle worm is an Internet mass mailer that harvests addresses… and sends itself to other recipients via e-mail,” Network Associates said.

The Virus Bulletin website said the worm did not appear dangerous, but should be monitored.